A major problem associated with sheet-feed devices is feeding papers of various weights and surface characteristics. In high-speed photocopying machines, facsimile machines, and the like, individual sheets are separated from a stack for use in the image-forming process, either as documents to be copied or blank sheets upon which an image may be formed. This separation is commonly achieved by applying a normal force to the exposed surface of a sheet in the stack, and drawing the sheet off the stack by friction. In separating and feeding the individual sheets, two types of malfunction are of most importance: multifeeding, in which more than one sheet is drawn from the stack at one time, and misfeeding, in which no sheets are successfully drawn from the stack. Very often these and similar problems are caused by the application of too much or too little normal force to a sheet being separated from the stack.
In current use, most sheet feeders are designed specifically for a particular type or weight of paper having a narrow range of characteristics. However, in practice, a machine may be exposed to a wide variety of sheets, ranging from extremely heavy paper (110 lb. card stock) to extremely light paper such as onion skin (9 lb. or 8 lb. bond). If a feeder is designed to handle light paper, it is not likely to handle heavy stock reliably. Similarly, a feeder designed to handle heavy paper may severely mutilate lightweight paper such as onion skin.
In addition to misfeeding and multifeeding, another problem of document quality caused by an excessive amount of force applied to the sheet is smearing. The normal force applied to a document for purposes of feeding and separating is likely to smear the ink on the sheet. Heretofore, the smear problem has been regarded as an inherent property of friction feeders, as the "window" of permissible normal forces, which will avoid the alternative risks of misfeeding and smearing, is so small.
To obviate the problems caused by inappropriate amounts of force in friction feeders, a number of schemes have been proposed. U.S. Pat. No. 4,475,732 to Clausing, et al., assigned to the assignee of the present application, discloses a system by which the normal force applied to the sheet on the stack to be separated is set at an initial high value, and is maintained at the initial high value until movement of the lead edge of the sheet to a certain location is detected. When movement of the sheet off the stack is detected, the normal force against the stack is momentarily relieved while the sheet continues to be drawn from the stack by take-away rolls. In this way, a large normal force insures that even the heaviest sheets will be drawn off the stack, but the large force is not applied long enough to damage the sheet.
U.S. Pat. No. 4,900,003 to Hashimoto discloses a sheet conveying apparatus in which a sheet to be conveyed is initially contacted by a pick-up roller, which is mounted on a reciprocating rocker member. The pick-up roller conveys the sheet to a separation roller, which urges the sheet against a separating plate. As described at column 7, lines 16-27 of the patent, the singularity of sheets being fed by the apparatus is achieved by maintaining a certain relationship of frictional coefficients among the sheet, the separation roller, and the separating plate.
U.S. Pat. No. 5,006,903 to Stearns discloses a sheet feeding apparatus wherein a feed belt disposed underneath a stack of sheets feeds the bottom sheet through a nip with a retard roller. The retard roller is braked by a motor which prevents motion of the retard roller when two sheets are in the nip.
Japanese patent 63-87436 discloses a medium (sheet material) delivering system in which a subroller disposed at the bottom of a stack of sheets is brought into contact with the bottom sheet. The subroller is coupled to a main roller by a delivery belt. The bottom sheet is moved from the stack by the delivery belt until it is caught by a pair of feed rollers, which continue moving the sheet. When the leading edge of the sheet is detected by a downstream sensor, the subroller is withdrawn from the bottom of the stack. As with U.S. Pat. No. 4,475,732 patent above, the operative principle is that the normal force against the stack is not maintained long enough to damage the sheet.
It is an object of the present invention to provide a self-adjusting friction feeder which automatically avoids the problems associated with an unsuitable amount of normal force applied to a sheet being separated.
It is another object of the present invention to provide a friction feeder which provides only the minimum necessary force required for a given type of sheet.
It is another object of the invention to provide a friction feeder which automatically adapts to whatever type of sheet is being used, without the need for external adjustments.
Other objects will appear hereinafter.